The recent portable electronic devices such as a foldable mobile phone and the like use a flex-rigid multilayer wiring board. FIG. 20 shows an example of such a conventional wiring board, in which rigid portions 500 and 520 are connected to a flexible portion 510 through a flexible substrate 544. Normally in the rigid portion 500, the flexible substrate 544 and pattern layers 504 and 506 on the surfaces of the rigid portions 500 and 520 are electrically connected to each other through a conductive layer in a plated through-hole 502 (cf. Japanese Patent Application Laid Open No. 90756 of 1993).
The conventional flex-rigid multilayer wiring board is manufactured in a manufacturing process shown in FIG. 21.
As shown in FIG. 21(a), prepared rigid substrates 540 and 542 and flexible substrate 544, and prepregs 546 and 548 for bonding the substrates 540, 542 and 544 to each other, are vertically lap-joined to each other. As shown, slits 550 are preformed in the rigid substrates 540 and 542 along the boundary between the rigid substrates 500 and 520 and flexible substrate 510.
Portions of the prepregs 546 and 548, corresponding to the flexible substrate 510, are cut off in advance. The flexible substrate 544 has formed on either side thereof a conductive pattern 511 which is protected by a coverlay 512.
The rigid substrate 540 and flexible substrate 544 are superposed one on the other as shown in FIG. 21(a), and joined to each other by pressing as shown in FIG. 21(b).
Then, the substrates thus lap-joined together are subjected to drilling, plating, patterning and the other process to form a wiring board having the pattern layers 504 and 506 formed on the surface thereof as shown in FIG. 21(c).
Note that at this stage, since the flexible substrate 544 in the flexible portion 510 is covered with the rigid substrates 540 and 542, the flexible substrate 544 will not be susceptible to any plating solution in the step of plating.
Thereafter, parts of the rigid substrates 540 and 542, corresponding to the flexible portion 510, are cut off along the slits 550 to form the flexible portion 510, thereby forming a flex-rigid multilayer wiring board as shown in FIG. 21(d).
The above conventional flex-rigid multilayer wiring board is manufactured following many steps such as placing a flexible substrate of polyimide film between rigid substrates of glass epoxy resin, glass polyimide resin or the like, and joining them together through prepregs or adhesive sheets by thermal compression bonding, then drilling, through-hole plating, photoresist application, etching and the like.
The above-mentioned conventional manufacturing process is not advantageous in that since the process after the thermal compression bonding as shown in FIG. 21(b) is complicated, the lead time for this process is relatively long. It is also disadvantageous in that in the step of cutting off a part of the rigid substrates (as shown in FIG. 21(d)), positioning in the direction of thickness has to be done accurately and cutting off the parts of the rigid substrates with a cutting tool or the like is often likely to result in scratching of the surfaces of the rigid substrates.
Also, the conventional flex-rigid wiring board is not advantageous in that it cannot be produced with any higher yield since it is formed from a combination of different materials, which leads to a displacement of them from each other due to a difference in thermal shrinkage between them during the thermal compression bonding, smear developed at the time of drilling, poor adhesion of the through-hole plating or the like.
Further, the conventional flex-rigid wiring board is disadvantageous in that the inter-layer connection is not highly reliable since the through-hole plate is likely to break away at the boundary between the through-hole formed through the flexible and rigid substrates and the substrates themselves because the substrates are different in thermal expansion coefficient from each other.
Moreover, the conventional flex-rigid multilayer wiring board basically includes a rigid substrate stacked on either side (front and back sides) of a flexible substrate and these substrates are electrically connected to each other through a through-hole formed through them.
In the above connection structure, signals are transmitted from one of the rigid substrates to the flexible substrate through the conductive layer plated on the wall of the through-hole but the conductive layer on the through-hole wall extending to the other rigid substrate is an unnecessary part through which the signals cannot be transmitted. The LC (inductance and capacitance) in the unnecessary part will cause a signal delay, and particularly a signal delay in a band of giga-level frequency, and a signal reflected at such an unnecessary part will disturb the signal waveform.
Also, since the aforementioned conventional flex-rigid wiring board is limited to a substrate configuration in which the flexible substrate is led out from the lateral side of the multilayer rigid substrate, it cannot produce any substrate configuration in which substrates are connected to each other with a high freedom of wiring.
Further, the flex-rigid wiring board is housed in a portable electronic device. To assure that the device housing the wiring board, even if dropped, will not easily be faulty, the wiring board itself is required to be able to withstand any dropping.